In response to relatively recent Federal and State "clean air" regulations, carburetor limiter caps have been used to restrict adjustment of fuel metering valves to prevent the excessive emissions of carbon monoxide and unburned hydrocarbons from an internal combustion engine.
Primarily, limiters have been used with conventional float bowl carburetors to control automobile exhaust emissions. More recently, their use has been extended to diaphragm-type carburetors in small engines simply to prevent gross misadjustment of fuel flow. However, future emissions regulations are expected to extend the role of limiter caps to control engine emissions.
Fuel flow within a carburetor is commonly metered during no-load of idle engine operation by a first "idle" needle valve and during part of full load operation by a second "main" needle valve. Typically, each valve has a threaded shank with an enlarged knurled head at one end and a conical or needle shaped valve control surface at the opposite end which is received within an opening in the carburetor body. To calibrate fuel flow, each valve is rotatably adjusted to axially extend or retract the valve control surface within a fuel passage in the carburetor until optimum fuel flow through the passage is achieved. Precalibration of the carburetor on a flow test bench may be performed prior to being assembled to an engine. After assembly to an engine, it is customary to adjust the metering valves, if needed, to fine tune fuel flow to actual engine demand. Subsequent overadjustment of fuel flow is prevented by affixing a limiter cap over the head of each valve.
These limiter caps generally consist of a cylindrical body having an opening at one end for axially receiving the valve head in tight fitting engagement to prevent easy removal and promote rotation of both in unison. A recess in the opposite end of the cap is provided to enable an adjustment tool to engage the valve directly or the cap alone to rotate both in unison to make fuel flow adjustments. Projecting radially outwardly from the cap body is an arm for abutting against a fixed stop, which may be a projection extending from the carburetor or an adjacent valve shank which may or may not have a cap, to limit valve rotation. Preferred limiter caps are disclosed in U.S. Pat. No. 5,322,645, the disclosure of which is incorporated hereby by reference.
There are times when it may be necessary to remove the limiter caps. For example, the valves may be incorrectly adjusted at the flow test bench in which case it is necessary to remove the limiter caps to correct the valve setting. Additionally, the carburetors may need to be reworked or modified as required by the customer at the engine manufacturing plant requiring re-setting the valves. Furthermore, field servicing dealers may need to remove the limiter caps to service or adjust the carburetor.